Method of using a tunneling diode in optical sensing devices

ABSTRACT

A method of fabricating a tunneling photodiode is presented comprised of the following steps: forming a p-well in an n-type substrate, forming a thin insulating layer over the surface of the p-type material, and then forming a thin n-type layer over the insulating layer. Preferably, the n and p type semiconductor material could be silicon and the insulating layer could be between about 30 to 40 angstroms of gate quality silicon dioxide. In other embodiments of the invention the materials of either electrode are either n or p-type semiconductors or metals.

This is a division of patent application Ser. No. 09/414,928, filingdate Oct. 12, 1999, Novel Optical Sensor By using Tunnel Diode, now U.S.Pat. No. 6,284,557, issued Sep. 4, 2001, assigned to the same assigneeas the present invention.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to photosensitive devices andmore particularly to an optical sensor having very low dark leakagecurrent.

(2) Description of Prior Art

Optical sensors are utilized extensively in modem technology. CompleteGuide to Semiconductor Devices, McGraw Hill, Inc., pp. 140-142, by KwokK. Ng, discusses buried-channel charge-coupled and peristalticcharge-coupled optical devices. Traditionally, photosensitive devicesare semiconductor diodes in which the light induced signal is related tothe passage of photon-generated electrons and holes through the electricfield region of the diode. To maintain low dark currents the diodes arereverse biased. The electrical field acts on thermally generatedelectrons and holes in the same way as on those that arephoton-generated. Usually, light induced signals dominate those in thedark when the electron-hole generation rate due to photons dominates thethermal electron-hole generation rate. At low light intensities thedifferences could be small and errors could result.

A junction photo-diode, shown in FIG. 1, is a typical conventionaloptical sensor. Region 2 is a p-type semiconductor and region 4 is athin n+ layer of the same semiconductor. As shown in FIG. 1, a reversebias, V, is applied and an ammeter, 6, measures the current. The energyband diagram of the biased junction photo-diode is shown in FIG. 2. Theelectric field is essentially confined to a depletion region of width,W, in the vicinity of the p-n junction, as shown in FIG. 2 Prior Art.When the doping density on one side of the junction is much larger thanon the other, the depletion region will predominately be on that side.The depletion width increases as the reverse bias increases. When lightis incident on the surface of the n+ region, 4, and if the photon energyis sufficiently high, electron-hole pairs are generated at a rateproportional to the light intensity. For those pairs generated withinthe depletion region, the electrons are swept to the n+ neutral regionand the holes to the p neutral region. Electrons generated in theneutral p region could diffuse to the depletion region and be swept tothe n+ neutral region. Effectively, this adds a width Ln, the electrondiffusion length in the p region, to W as the total width whereelectrons are swept by the field to the neutral n+ region. Similarly,W+Lp, where Lp is the hole diffusion length, is the width where holesare swept by the field to the neutral p region. The photocurrent is thusproportional to W+Ln+Lp times the photon induced pair generation rate.In the absence of light the current is essentially proportional toW+Ln+Lp times the thermal generation rate. Thus at low light intensitiesthe photocurrent need not dominate the dark current.

It is important that light penetrate to the vicinity of the depletionregion so that significant pair generation occurs where the createdelectrons and holes can be acted on by the field and thus contribute tothe current. Therefor the n+ region, 4, must be thin enough to allow forthis penetration. A junction photodiode can also have a thin p regiondisposed over an n region, with the light incident on the p region.

U.S. Pat. No. 4,965,212 to Aktik shows a junction photodiode comprisedof a thin p+ type hydrogenated amorphous silicon layer disposed on alayer of n-type hydrogenated amorphous silicon. Also shown is aphotosensitive diode where the p+ type layer is replaced by a thinmetallic layer. Another photosensitive device utilizing hydrogenatedamorphous silicon is described in U.S. Pat. No. 5,844,292 to Thierry.There the device is a p-i-n diode with the p-type, intrinsic and n-typelayers being composed of hydrogenated amorphous silicon. A p-i-nphotodiode operates similarly to a p-n junction photodiode, theintrinsic layer of the p-i-n diode acts in the same way as the depletionregion of the p-n diode.

U.S. Pat. No. 5,114,866 to Ito et al. shows an avalanche photodiode inwhich an additional doped region is added to prevent edge breakdown. Anavalanche photodiode is essentially a junction photodiode operated athigh reverse bias where avalanche multiplication takes place. To obtainspatially uniform multiplication, edge breakdown must be eliminated.

U.S. Pat. No. 5,260,225 to Liu et al. shows a method for fabricating aninfrared bolometer. The method utilizes oxide and silicon nitride layersto provide a location for the active layer, an appropriately dopedpolysilicon layer.

SUMMARY OF THE INVENTION

Accordingly, it is a primary objective of the invention to provide aphotosensitive device with minimal dark current and where the ratio ofcurrent in light to dark current increases with increasing bias. Thuseven at low light intensities the photon-induced current can be made toexceed the dark current.

The objectives of the invention are achieved by using a tunnel diode,with a wide band gap insulator providing the tunneling barrier, as thephotosensitive device. For an appropriately high and thick potentialbarrier, currents in the dark at moderate bias are very small. Incidentlight energizes tunneling electrons, essentially reducing the barrierenergy and increasing the tunneling probability. The affect of the biasis more pronounced on this essentially reduced barrier. Thus, whereas inconventional photodiodes the electric field act on optically andthermally generated current carriers in the same way, for tunnelingphotodiodes the affect of the field is more pronounced on the opticallyenergized carriers.

According to a preferred embodiment of the invention a tunnelingphotodiode is fabricated by forming a p-well in an n-type substrate,forming a thin insulating layer over the surface of the p-type material,and then forming a thin n-type layer over the insulating layer.Preferably, the n and p type semiconductor material could be silicon andthe insulating layer could be between about 30 to 40 angstroms of gatequality silicon dioxide.

In other embodiments of the invention the materials on either side ofthe insulator could be either n or p-type semiconductors or metals. Theinsulating layer should exhibit very low leakage in the dark and bereliable even for the thin layers, usually less then 100 angstroms,required in this invention. Gate quality insulators generally meet theserequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, forming a material part of thisdescription, there is shown:

FIG. 1 (Prior Art) shows a conventional junction photodiode.

FIG. 2 (Prior Art) presents a band diagram of a conventional junctionphotodiode.

FIG. 3 shows a tunnel photodiode according to a preferred embodiment ofthe invention.

FIG. 4 presents a band diagram of a tunnel photodiode according to apreferred embodiment of the invention.

FIG. 5 is a graph of the current versus voltage, with and without light,measured on a tunnel photodiode fabricated according to a preferredembodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3 there is shown the structure of a tunnel photodiodeaccording to a preferred embodiment of the invention. As in a junctionphotodiode, a p-well, 2, is formed in an n-type substrate, 8, wherepreferably the semiconductor is silicon. A p-well is a preferred methodto provide isolation from other devices in an n-substrate. Proceduresfor forming p-wells, and also n-wells, are well known to those versed inthe art. A thin insulating layer, 10, preferably a layer of thermallygrown silicon dioxide about 30 to 40 angstroms thick, is disposed overthe surface of the p-well. An n-type polysilicon layer, 4, is thendeposited over the silicon dioxide layer. The thickness of thepolysilicon layer should not appreciably exceed the absorption length ofthe light within the wavelength range of interest. Techniques forforming the oxide and polysilicon layers are well known to those versedin the art.

A band diagram of a tunnel photodiode according to this embodiment isshown in FIG. 4. For reverse bias electrons tunnel predominately fromthe p-well, 2, toward the n-type polysilicon, 4. In the dark a typicaltunneling path is the direct tunneling path 12, where an electrontunnels from the p-well conduction band to the conduction band of thepolysilicon. When light is incident on the polysilicon surface, photonseasily penetrate the polysilicon layer, which is thinner than anabsorption length of the light, and even more easily penetrate thetransparent thin oxide layer. Photons, 14, interacting with electrons ofthe p-well raise their energy and thus when these electrons tunnel thebarrier heights are reduced by the energy gained, as indicated inelectron path 16. As shown in FIG. 5, the current with light, 18, is byfar more strongly increasing with bias than the dark current, 20.Therefor the current in light of a tunnel photodiode can always, byadjusting the bias, be made to dominate the dark current. The tunnelphotodiode used has a 35-angstrom thick silicon dioxide layer thermallygrown on a p-well and a thin n-type polysilicon layer deposited on theoxide. For the voltage range covered the dark current is negligiblysmall; whereas the current with light is already in the nanoampere rangeat low bias and increases exponentially with bias.

In other preferred embodiments of the invention the bottom electrode canbe a p-type substrate or a p-type semiconductor deposited on an n orp-type semiconductor substrate or on an insulating substrate. Depositionand isolation techniques, if required, are well known to those versed inthe art. If the bottom electrode is to be the injecting electrode,absorption lengths of materials used should be small enough so thatlight is absorbed strongly near the insulator interface resulting inlarge numbers of energized electrons that pass through the insulatorwith high probability; otherwise this is not desirable. The bottomelectrode layer thickness range is not dictated by light detectionrequirements. It is important that the conductivity and thickness belarge enough so that the electrode does not introduce significantresistance.

The insulating layer disposed over the bottom electrode can either bethermally grown or deposited. Wide bandgap insulators are to be used forthe insulating layer since large barrier heights are required for lowdark currents. A lower boundary for the insulating layer thickness isimposed by reliability requirements. The insulator field at the appliedbias during operation should not be so large that dielectric breakdowncould occur, during the desired life of the device, with an intolerablelikelihood. For gate quality insulating films, such as thermal siliconoxide films and LPCVD silicon oxide, silicon nitride and oxynitridefilms, high reliability is achieved when the field in the insulatinglayer is less then about 5 E6 volts per centimeter. Thus, for example,with a bias of 1 volt and a built-in potential of 0.75 volts, theinsulating film thickness should be greater than about 35 angstroms.Another criteria that gives rise to a lower boundary is the requirementof low dark current, but the boundary obtained in this way is usuallylower than that obtained from the reliability criteria. An upperboundary for the insulator layer thickness is imposed by detectabilityrequirements. The thinner the insulating layer the lower is the lightintensity of a given wavelength that can be detected at the applied biasand the longer the wavelength the lower is the signal. Therefore, theminimum signal required at the lowest light intensity desired to detect,at the longest wavelength of interest, can determine the largestacceptable insulating layer thickness.

The upper electrode, in the preferred embodiments, can be a depositedlayer of n or p-type polysilicon or other semiconductor, or a depositedmetal layer. Semiconductor materials that can be used include silicon,poly and amorphous-silicon, hydrogenated silicon, germanium, and galliumarsenide; metals such as gold, chromium, aluminum, tantalum, tungstenand platinum are also appropriate electrode materials. The thickness ofthis layer should not be greater than the absorption length of the lightso that a significant number of electrons will be energized near theinsulator interface to pass through the barrier. Barrier heights varyfor different electrode materials on the same insulator. The larger thebarrier height, the larger the bias or the thinner the insulator layerthat is required for a given signal. Also the built-in potential isdetermined, to a large extent, by the barrier heights at bothelectrode-insulator interfaces. As with the bottom electrode, theconductivity and thickness should be large enough not to introducesignificant resistance.

Other preferred embodiments utilize n-type semiconductors as bottomelectrodes; either an n-well, a deposited layer or an n-type substrate.Metallic layers on semiconductor or insulator substrates are appropriateas bottom electrodes according to preferred embodiments of theinvention. The insulator and top electrode layers in these preferredembodiments are as described for the preferred embodiments with a p-typesemiconductor as the bottom electrode.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of using a tunnel photodiode comprising:providing a substrate and forming a p-type semiconductor layer on saidsubstrate to be a bottom electrode; forming an insulating layer on thebottom electrode; forming a conducting layer over the insulating layer,whose thickness is less than about the absorption length of light to bedetected, to be a top electrode; forming other elements andinterconnections of an optical sensing device; whereby a tunnelphotodiode, having an insulating barrier layer disposed between top andbottom electrodes, is formed and using said photodiode as a photonsensing element of an optical sensing device.
 2. The method of claim 1,wherein said bottom electrode is a semiconductor or metal layer.
 3. Themethod of claim 1, wherein said insulating layer is a silicon oxide,silicon nitride or silicon oxynitride layer.
 4. The method of claim 1,wherein said top electrode is a semiconductor or metal layer.